Patent GB-1437765-A (GB)
Title: Hydrostatic transmissions
IPC: F04B49/00
Filed: 1974-05-17
Abstract: 1437765 Hydrostatic transmission control SULZER BROS Ltd 17 May 1974 [17 May 1973] 22136/74 Heading F2W In a hydrostatic transmission wherein a motor M is supplied with pressure liquid from a variable displacement radial piston pump VP and selectively from fixed displacement pumps FP1, FP2, a control valve 16 which selectively connects the pump FP1, FP2 to the motor M is interconnected with the displacement adjusting mechanism 12 of the variable displacement pump to permit connection or disconnection of the pumps FP1, FP2 to or from the motor M only when the displacement adjusting mechanism 12 is in its maximum or minimum displacement positions. The interconnection comprises a pin 14 carried by the displacement adjusting mechanism, 12 and engaging with a cam slot 15 in a rotary spool of the valve 16. The displacement adjusting mechanism 12 and rotary valve 16 are controlled by pressure liquid supplied from the working circuit through valves 21, 22 controlled by a lever 20. Rotation of the lever 20 in a clockwise, speed increasing direction, depresses the spool of the valve 21 to supply control pressure liquid P ST  to a line b. Pressure in the line b is supplied through a change-over valve 23 and line d to the lower end of the displacement adjusting mechanism 12 which is moved upwardly to increase the displacement of the pump VP. When the pump displacement reaches its maximum value the pin 14 engages a circumferential portion of the cam slot 15 to permit rotation of the valve spool 16. The pressure in the line b additionally acts on a vane (18) of the rotary valve spool 16 to rotate the latter through a predetermined angle permitted by the pin 14 and cam slot 15. The valve 16 then supplies pressure liquid through a line a to the end of the change-over valve 23 the spool of which is moved left against the bias of a spring 24 to connect the line d to vent and to connect the line b to a line e supplying the upper end of the displacement adjusting mechanism 12 which moves down to reduce pump displacement to zero. The pin 14 and slot 15 then permits further rotation of the valve spool 16 to connect the fluid pump FP1 to the motor M. The further rotation of the valve spool also cuts off the pressure liquid supply to the line a whereupon the change-over valve 23 returns to the position shown in which pressure liquid is supplied to the lower end of the displacement adjusting mechanism 12 for further increase in displacement of the pump VP and increase in output speed. When the pump VP reaches its maximum displacement the valve spool 16 is permitted further rotation to connect a second pump FP2 to the motor M the displacement of the pump VP being returned to zero in a manner as described above. The displacement may be reduced by moving the lever 20 in an anticlockwise direction to supply the control pressure P ST  through the valve 22, line c and change-over valve 23 to the upper end of the displacement adjusting mechanism 12. The fixed displacement pumps FP1, FP2 being sequentially disconnected from the motor M by the valve 16 in a manner similar to that described above each time the pump VP reaches a minimum displacement value. The control pressure P ST  supply to the valves 21, 22 and to the rotary valve 16 is supplied from the working circuit high pressure line 31. A makeup pump ZP supplies pressure liquid to the low pressure side of the fixed and variable displacement pumps. Restrictions 38 prevent rapid movement of the spools of the valves 21, 22.